A torch for electric-arc work, such as a plasma cutting torch, usually comprises an electrode, made completely or partly of an emissive material, which electrode has a generally cylindrical or frusto-cylindrical shape, a nozzle placed coaxially with respect to the electrode and forming a diaphragm in the path of the plasma arc, a circuit for internally cooling the torch, especially the electrode, and one or more circuits for delivering a plasma gas into a chamber bounded, on the one hand, by the electrode and its support and, on the other hand, by the internal part of the nozzle and of its support.
In operation, the electrode is connected to one of the poles of a power supply while the nozzle is connected to the other pole of the power supply.
After part of a plasma gas stream, flowing between the lower end of the electrode and a gas ejection channel made in the nozzle, has been ionized, a first electric arc is generated, thus forming an arc plasma column starting at the base of or near the electrode and being drawn through the channel of the nozzle to the outside and as far as the material to be cut or welded, for example.
Depending on the type of use of the torch, the gas jet is brought up to a suitable power and maintained between the electrode, forming the cathode for example, and the nozzle, therefore forming the anode, throughout the welding or cutting operation for example, or, depending on the situation, before increasing the power, the gas jet is transferred to the workpiece, by closing up and electrical switching, the workpiece then forming the anode and the nozzle possibly being electrically disconnected.
Such torches and such processes are described for example in documents EP-A-599,709, EP-A-573,330, U.S. Pat. No. 5,597,497, WO-A-96/23620, U.S. Pat. No. 5,451,739, EP-A-0,787,556, FR-A-2,669,847 and FR-A-2,113,144.
In some cases, especially for technical or constructional reasons, the plasma gas in the chamber bounded by the electrode and the nozzle is delivered in an injection plane approximately perpendicular to the axis of the electrode. Thus, the gas may be injected into the chamber as a ring centered on the axis of the electrode, via a continuous and circular slit.
In another situation, the gas may be injected into the chamber via a piece having the shape of a ring within which calibrated holes are made, the axes of the holes converging on and intersecting the axis of the electrode.
Likewise, a TIG welding torch comprises a generally pointed or sharpened tungsten electrode, the electric welding arc being established between the electrode and the workpiece to be welded.
However, a problem arises that has not been satisfactorily solved hitherto, namely the problem of rapid wear or deterioration of the electrode during use of the plasma torch or the TIG torch in a welding, cutting or similar process.
In fact, the lifetime of an electrode, particularly a pure or alloyed tungsten electrode, depends on many parameters, such as the power of the electric arc, the nature and flow rate of the gas and the operating cycle of the torch.
Now, rapid electrode wear is very costly from an industrial standpoint given that it entails more frequent maintenance operations in order to replace the worn pieces or to resharpen the electrode in the case of a TIG torch, this only being possible after dismantling the torch and therefore involving frequent and inopportune production stoppages. This therefore also increases the overall cost of the welding process.
Furthermore, electrode wear causes, moreover, an appreciable reduction in the performance of the torch, leading to a degradation in the quality of the work produced, for example a reduction in the quality of the cut obtained.
Document FR-A-2,251,153 emphasizes the problem of tungsten electrode oxidation and recommends, in order to try to solve the problem, adding from 0.1 to 1% of hydrogen to argon or helium in order to obtain a plasma gas mixture.
Moreover, document JP-A-06,039,554 describes a shielding gas, for welding galvanized steel, which contains up to 7% oxygen and also carbon dioxide.